Set-back Distances

A 1986 U.S. Geological Survey (USGS) publication by Garnett Williams compiled and compared stream geometry measurement data from a number of sites distributed worldwide. The results of the study found that a majority of bankfull width to meander-belt width ratios across all sites tended to fall within the range of two to ten with the highest number of instances occurring at or near a ratio of six. This suggests that natural channels generally utilize an area of approximately six times their bankfull width as they attempt to establish equilibrium.

Based on these findings, in the 1990’s the Vermont Fluvial Erosion Hazard (FEH) Program developed a fluvial erosion hazard mapping procedure in  which an automatic initial set-back width of three bankfull widths is applied to either side of the defined “meander centerline.” As an added factor of safety, an additional bankfull width is added to each side of the meander centerline for a total of eight times the estimated bankfull width to create what is defined as the suggested “total (erodible) corridor width” (see images below).  Based on the successes of the Vermont program, a number of states and municipalities have adopted  this technique. A similar method is also being used throughout the European Union.

This mapping technique, when paired with tools including Regional Channel-Dimension Curves developed for the State of Indiana and the U.S. Geological Survey’s (USGS) online software tool StreamStats, large areas of preliminary mapping can be produced in a relatively short period of time with little or no field investigation. However, the Indiana FEH team would like to emphasize the preliminary nature of this step. It must be understood that this method alone does not take into consideration the suite of other variables that may affect an area’s susceptibility to fluvial erosion hazards.







Williams, G.P., 1986. River Meanders and Channel Size. Journal of Hydrology 88: 147-164.

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